Tanks that store liquefied gasses maintained at a temperature substantially below ambient or atmospheric temperatures and at relatively low pressures are insulated to maintain the fluid at the desired temperature and/or pressure. For example, tanks which store liquefied gasses at a low temperature and pressure are insulated to reduce the liquid to gas phase transformation within the tank to a low level. Referring to FIG. 1, an example of a cryogenic tank 10 is shown. The tank 10 includes a primary liquid container 1, which holds the fluid, and a secondary liquid container 2 located around the primary liquid container 1. Tank insulation 6, 8, is located between the primary liquid container and the secondary liquid container. The tank 10 also includes a roof 3 located above the stored liquid and separated from the liquid by an insulated second roof 5 that may be suspended from the first roof 3. The space 7, or warm vapor space 7, between the roofs (i.e., between roof 3 and insulated second roof 5) or between the roof 3 and secondary liquid container 2 contains warm (relative to the stored liquid) product vapors and allows the first roof 3 to remain near ambient temperature.
Process pipe 12 carrying fluids e.g., liquefied gas and cold vapor, to and from the primary liquid container protrudes from an opening in the roof 3 or a sidewall of secondary liquid container 2 of the storage tank 10. The process pipe 12 may be one continuous pipe, or may include a number of pipe segments. The connection into the secondary container 2 or roof 3 must maintain the structural and thermal integrity of the tank 10. In order to maintain proper temperature requirements of the warm first roof 3, a pressure containing connection 20 is located at the opening and positioned around the cold process pipe 12 located in the opening. This connection 20 completes the container pressure boundary, accommodates piping loads to the tank 10, acts as a vapor barrier for the insulation, and transfers the thermal gradient between the cold pipe and the warm container 2. The section of the connection where the thermal gradient occurs is referred to as the thermal distance piece (TDP).
Referring to FIG. 2, an example of a prior art TDP assembly 20 is shown. Conventionally, a portion of the TDP 20 is exposed to ambient conditions outside tank roof 3 to provide beat to the TDP 20. The TDP 20 includes a sleeve 23 positioned around a portion of the process pipe 12 located within an opening 31 of the tank roof 3. The sleeve 23 includes an annular top plate 24 welded to a top end of the sleeve 23. An inner circumferential surface of the annular top plate 24 is welded to an outer circumferential surface of the process pipe 12 to connect the sleeve 23 to the process pipe 12. The sleeve 23 is welded to the tank roof 3. Welding the sleeve 23 to the tank roof 3 creates a direct load transfer between the TDP 20 (including the process pipe 12) and the tank roof 3. Additionally, the welded connection between the sleeve 23 and tank roof 3, the welded connection between the top plate 24 and sleeve 23, and the welded connection between the top plate 24 and process pipe 12 create a vapor tight connection and prevents vapors located in the tank from exiting the tank and ambient moisture outside the tank from entering the TDP 20 and the tank 10.
Insulation 21, e.g., granular insulation, fiberglass, foams, or other insulating materials known in the art, is located between the process pipe 12 and the sleeve 23. Because the sleeve 23 is welded to the roof 3 at the tank site, insulation is usually installed after the sleeve 23 is welded to the roof 3, as most insulation materials are sensitive to high temperatures. Those assemblies that occasionally did install the insulation material in the shop were well known in the industry of having shorter industrial lifespans due to thermal insulation cracking. Were the insulation 21 installed prior to welding the sleeve 23 to the tank roof 3, the high heat of the welding process would cause portions of the insulation 21 to melt and/or create voids within the insulation 21. Any voids in the insulation 21 make the insulation 21 less effective and allow frost to form along an outer diameter of the sleeve 23 proximate the location of the void.
Continuing with the above example of prior art, the insulation 21 is installed through a plurality of circular openings 25 in the top plate 24 or a plurality of openings 26 in the sleeve 23. Conventionally, a blower or jet pump provides positive pressure to blow insulation into the annular space between the sleeve 23 and the process pipe 12. Thus, the type of insulation 21 selected to be installed should be able to be installed through openings 25. Once the insulation 21 is installed, the openings 25 are sealed. Because the openings 25, 26 to the insulation 21 are readily accessible, in the event that the insulation 21 fails, a worker is able to reinstall and/or repair insulation 21 without removing the entire TDP assembly from the tank roof 3.
However, the direct contact between the top plate 24 and the process pipe 12 conducts heat away from the upper end of sleeve 23, reducing the temperature of the upper end of the sleeve 23 significantly. The exposure of the top plate 24 and areas of the sleeve 23 proximate the top plate 24 to moisture in the atmosphere can cause condensation and ice to form around the TDP 20, which reduces the efficiency of the TDP, adds to the required maintenance of the area around the TDP 20, impedes access to the TDP 20, and creates a potential safety hazard. Accordingly, there is a need for a TDP assembly that reduces and/or eliminates the formation of ice on the TDP.